How to separate locked areas?
For easy understanding, let's just call the areas in your image as "Left" and "Right"; and "Middle" for the new split area:



A Three-click Way:


Right Click on the border of the right area;
Choose "Split Area", move the mouse to the area you want to split;
Click and drag on the little triangle at the upper-right corner of the left area towards the middle area, to join them as a new one. Release the mouse button.


As shown below:





A Two-Click Way:


Click and drag on the little triangle at the upper-right corner of the left area;
Click and drag on the little triangle at the upper-right corner of the middle area towards to the right area, then turn back to left. Release the mouse button.